1. Technical Field
The present invention relates generally to thermal imaging systems, and in particular to an aspherical lens assembly in an infrared imaging system imager that minimizes the number of components and space required to implement the imager.
2. Discussion
Scanning thermal imaging systems are used in a variety of applications, including surveillance systems and target detection/recognition systems. Such systems typically incorporate a telescopic lens assembly coupled to a scanner. The scanner scans energy from a scene through an imager lens assembly onto a detector array having a plurality of photoelectrically responsive detector elements perpendicular to the scan direction. Each of these detector elements provides an electric signal proportional to the flux of infrared light on the particular detector element. Electric signals generated from the detector elements are subsequently processed by system sensor electronics to create an image that is displayed on a system output device. To improve sensitivity, some of these systems incorporate detectors parallel to the scan direction. The output of these detectors are delayed in time from each other such that, ideally, the scanned image is output simultaneously on all of the parallel detectors. The delayed outputs are then summed (integrated). This process is referred to as time delay and integrate (TDI).
In the above-mentioned thermal imaging systems, the imager must focus the thermal energy from the detected scene sharply onto the detector array to ensure clarity of the video signal output on the system output device. In conventional imager systems, multiple lenses were implemented to effect the focus of the detected scene energy onto the array. While such multi-lens imagers provide adequate focusing of the detected scene energy onto the array, there are advantages to further improvement in the art for the following reasons.
First, each element in an imager is required to be manufactured to certain tight tolerances and then incorporated into the imager to other tight tolerances on such variables as centration and tilt. The elimination of elements then greatly reduces the complexity of fabrication of the imager. Additionally, system weight is also reduced.
Second, multiple lens imagers require a great deal of system volume to provide for the lens elements and mechanisms which hold them in place. As present-day thermal imaging system parameters continue to decrease in size, it would be desirable to minimize image lens assembly volume requirements.
Third, imager expense is increased in proportion to the number of lenses implemented in the system. The above-mentioned lens materials have a high associated cost. In addition, lenses formed from commonly-used brittle materials such as AmTir1 are highly susceptible to breakage and must be replaced when broken, thereby adding further cost to the system.
What is needed then is a thermal imaging system imager for focusing detected scene energy onto a system detector array that is implemented with a minimum number of elements, thereby reducing system cost and complexity while at the same time maintaining necessary optimum performance characteristics.